This invention relates generally to demultiplexing signals encoded in a light beam and, more particularly, to optical devices for demultiplexing different wavelengths of laser radiation.
Lightwave communication over fiber optics is attractive because it offers tremendous bandwidth, low noise and low attenuation in transmission over considerable distances. Conventional fibers offer three low attenuation and high capacity transmission bands passbands in the near infra-red at approximately 0.85, 1.3 and 1.5 microns. Each such passband in a typical fiber is about 200 nm wide between half-power points and therefore provides 26,000 gigahertz transmission capacity, enough to accommodate hundreds if not thousands of discrete wavelengths or transmission channels. While the transmission capabilities of fiber networks are impressive, communication of information ultimately demands that a particular channel be established between a given transmitter and a given receiver, if not permanently then at appropriate times.
Heretofore the problem of associating one receiver with one transmitter has been approached mainly by using tunable transmitters, tunable receivers, or both. An alternative approach is disclosed in U.S. Pat. No. 5,040,169 to Guerin et al in which an illustrative four modulated wavelengths W1, . . . , W4 are demultiplexed in a xe2x80x9cswitching fabricxe2x80x9d at a point 30 into two wavetrains W1, W2 and W3, W4 and then, at subsequent points 36, 38 further demultiplexed into four distinct output positions 41-44 in a horizontal plane, at each of which only one modulated wavelength appears. The same output position also appears in one of four vertical planes. While the duplication of output positions may serve a useful purpose, this redundancy incurs extra cost.
It is known that a multiple wavelength light beam may be split into physically distinct positions with a Fizeau interferometer which maps optical frequencies into a spatial distribution of fringe intensities. The fringes may be detected with photodiodes and then subjected to signal processing for estimation of the position of the Fizeau fringe as a function of signal level and fringe width. While a tunable Fabry-Perot interferometer in which mirror spacing can be varied can be used to scan a light beam in order to obtain one wavelength at a time, this may involve an intolerable delay in obtaining the desired wavelength or group of wavelengths one channel at a time.
A compact wavelength division demultiplexer device according to an illustrative embodiment of the invention comprises two orthogonally cascaded interferometers which are interposed at a point in a wideband light path at which it is desired to individually resolve (demultiplex) the multiple information signals carried by the light beam into spatially separated discrete wavelengths or channels. Thus, for a given transmission band, say one in the 1300 nm region, where there may be n sub-bands each carrying m information channels, the cascaded arrangement would resolve any of the nxc3x97m channels. The incoming beam of laser light is collimated and applied to the first interferometer. The free spectral range of the first interferometer is determined by the total bandwidth required by the nxc3x97m channels and its finesse (a measure of the interferometer""s channel selectivity) is determined by the resolution needed to spatially separate each of the n sub-bands from one another. The separation is determined by the amount of crosstalk which can be tolerated between adjacent sub-bands. The light leaving the first interferometer remains collimated and enters the second interferometer which is in line with the first interferometer and whose wedge angle is at 90 degrees to the plane of the wedge angle of the first interferometer. The two interferometers are oriented such that the dispersion of light with wavelength from the second interferometer is orthogonal to the dispersion of light with wavelength from the first interferometer. The free spectral range of the second interferometer is determined by the number (m) of channels within a sub-band and its finesse is determined by the need to resolve the m channels from one another. The separation here is determined by the amount of crosstalk which can be tolerated between adjacent channels. The collimated beams emerging from the second interferometer in which the location of each fringe depends on its wavelength are imaged upon a detector array or a bundled array of individual fibers, each of which ultimately terminates in an individual detector (not shown). In either event, each detector is uniquely associated with a specific wavelength of the free spectral range of the second interferometer.